1. The Field of the Invention
This invention relates to a plastic dipstick used to measure the depth of liquid nitrogen in a cryogenic container. More particularly, the present invention provides a flexible plastic stick which permits rapid and easy reading of a frost line to measure the depth of the liquid nitrogen.
2. Description of the Prior Art
Cryogenic containers are used to keep biological materials frozen in smaller containers with liquid nitrogen as the refrigerant. These devices are known as liquid nitrogen refrigerators and they are large metal vacuum bottles with an incredibly efficient insulation system. The key element of maintaining this efficiency is the vacuum space which is almost totally devoid of air molecules and an insulating neck tube between the inner and outer shells of the refrigerator. One type of refrigerator is made by the Linde division of Union Carbide and available from the American Breeders Service in DeForest, Wisconsin. Another type of refrigerator is sold by the Minnesota Valley Engineering in New Prague, Minnesota.
The biological material to be refrigerated is placed inside a circular container which is then inserted through the neck of the refrigerator and into the refrigerator containing the liquid nitrogen. Since liquid nitrogen has a boiling point of -195.83.degree. C. and a melting point of -209.86.degree. C., the temperature in the container is at least lower than about -195.degree. C.
These refrigerators come in various sizes and are designed for various holding periods such as, for example, four weeks, eight weeks, or sixteen weeks. However, all refrigerators eventually will fail and so one cannot be sure a given refrigerator will achieve its designated holding time between refills. Furthermore, as with any new equipment, a new refrigerator may also be faulty. Since refrigerator failures are usually unexpected and result in serious loss, it has been recommended that the level of the liquid nitrogen be measured by a dipstick each day. It is not possible to look inside the container to see the level of the liquid nitrogen since the top neck of the refrigerator usually has a small opening. Instead, dipsticks have been used which are inserted down to the bottom of the container and left for a short period of time. The portion of the stick immersed in the liquid nitrogen becomes exceedingly cold. The stick is then removed from the refrigerator and held in the surrounding atmosphere where the moisture in the air will condense and form a frost on the portion of the stick that was immersed in the liquid nitrogen.
One of the problems involved with this measuring technique is the upward movement of the front line during the period of time required for the moisture in the air to condense and form frost on the stick. While one waits for sufficient moisture in the air to transfer down and form frost on the stick, the latent heat, in the portion of the stick that was above the liquid nitrogen level, flows down to the very cold region below so the temperature in the upper region drops below 0.degree. C. Thus the region of the stick that was above the liquid nitrogen level now becomes cold enough to cause additional moisture to freeze and form frost in this higher region. As a result, a frost line forms higher up on the stick than at the true level.
A wooden dipstick has a large amount of mass with a corresponding large amount of latent heat. When they are inserted in the liquid nitrogen they transfer their latent heat to the liquid which enhances the further evaporation of the liquid nitrogen.
Another form of dipstick used is a metal rod such as a round, black clothes hanger wire. After the frost line has formed on this rod, it is then necessary to use a ruler to measure the length of the frost portion. Similarly a thin rod made of black plastic material has been used to measure the depth of the liquid nitrogen but it also requires a separate measuring ruler to measure the length of the frost layer. Dipsticks made out of aluminum have not been found effective because aluminum is such a good conductor that is does not form a good frost line.
3. Objects of the Invention
It is an object of the present invention to provide a low mass measuring stick that can be immersed into liquid nitrogen in a cryogenic refrigerator and which permits the rapid formation of a frost line to measure the depth of the liquid nitrogen.
It is a further object of the invention to provide a flat measuring dipstick having a graduated scale thereon so the point at which the bottom of the frost line occurs can be easily measured by reading the scale on the dipstick.
It is a further object of the invention to provide a dipstick which can be rapidly contacted with a large amount of air after the dipstick is withdrawn from the liquid nitrogen-containing refrigerator so the moisture in the air can be rapidly deposited on the cold portion of the stick as frost.
It is another object of the invention to provide a measuring stick which can be made of a flexible material so that the stick can be wound up into a circular configuration to provide easy storage and handling in a small space and to have an opening at one end to permit hanging of the stick when not in use.